Lactation is a reproductive process, which is unique to mammalian species and essential for their survival. Although not critical for human survival, breast milk is considered the optimal nourishment for newborns. Prolactin (PRL) is an anterior pituitary hormone which influences many aspects of lactation including maternal behavior, mammary gland development/ differentiation and the production of milk proteins. Elevated PRL levels are essential for adequate lactation. The initial signal for increased PRL secretion is the suckling of the newborn. The signal is transmitted by a specific neural pathway through the spinal cord and brainstem to the hypothalamus where it acts on neurons to increase PRL release and maintain competent lactation. Tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus provide the primary inhibitory influence on PRL release. TIDA neuronal activity is suppressed during lactation, contributing to the elevated levels of PRL. The endogenous opioid peptides are an important component in the neuronal network to translate the suckling stimulus into increased PRL secretion. These opioid peptides contribute to the suppression of TIDA neuronal activity during lactation. However, we do not understand the cellular/molecular processes occurring in the dopaminergic neurons in response to the suckling stimulus and how opioid peptides interact with the dopaminergic neurons to influence these processes. The first specific aim will assess the cellular regulation of tyrosine hydroxylase in TIDA neurons by evaluating the phosphorylation state of tyrosine hydroxylase protein and transcriptional rate for tyrosine hydroxylase gene in response to the suckling stimulus. The second specific aim will evaluate the co-localization of particular opioid receptor subtypes within the TIDA neurons and opioid receptor expression in response to the suckling stimulus. The third specific aim will investigate opioid receptor subtype(s) involved in the control of PRL secretion and TIDA neuronal activity by using selective opioid receptor antagonists. Overall these studies will contribute to our understanding of the complex neuronal pathways involved in maintaining lactation.